A lxml security update has been released for Debian GNU/Linux 9 LTS to address a XSS issue.

DLA 2606-1: lxml security update

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Debian LTS Advisory DLA-2606-1 debian-lts@lists.debian.org
  https://www.debian.org/lts/security/ Thorsten Alteholz
March 24, 2021   https://wiki.debian.org/LTS
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Package : lxml
Version : 3.7.1-1+deb9u4
CVE ID : CVE-2021-28957

An issue has been found in lxml, a pythonic binding for the libxml2 and
libxslt libraries.

Due to missing input sanitization, XSS is possible for the HTML5
formaction attribute.

For Debian 9 stretch, this problem has been fixed in version
3.7.1-1+deb9u4.

We recommend that you upgrade your lxml packages.

For the detailed security status of lxml please refer to
its security tracker page at:
  https://security-tracker.debian.org/tracker/lxml

Further information about Debian LTS security advisories, how to apply
these updates to your system and frequently asked questions can be
found at:   https://wiki.debian.org/LTS